A High Stability Clock Generator

For some special correlation experiments the use of a high stability clock generator offers significant advantages over the use of standard stability clock generators for generating the sampling time clocks of the correlator electronics. To visualise the practical difference of such two clock generator types, the three main sources of frequency uncertainty of these must be understood. A well selected quartz based clock generator usually shows an absolute uncertainty from its design frequency of +/- 10 ppm as well as a temperature dependent frequency drift of +/- 25 ppm over the operating temperature range (usually 0 ... 50°C) or a drift in the order of 1 ppm /°C (however, this drift is not a linear function). Furthermore, ageing of the quartz crystal will lead to an additional drift of 1 ... 2 ppm per year in the absolute frequency. If one was building an alarm clock with such a clock generator and operate it under standard room temperature conditions, it will give the time incorrectly by approximately 8 minutes per year. Not bad indeed, but not really precise as well.

In contrast, a high stability, temperature compensated quartz clock generator will show approximately +/- 0.5 ppm  initial frequency uncertainty, a temperature dependent drift of +/- 1 ppm over the operating temperature range (again 0...50°C) and ageing of less than 1 ppm / year. The same alarm clock now build with this clock generator would give the time correctly to better than a minute already. If the initial frequency uncertainty and ageing of the quartz crystal could be compensated for to, say 0.25 ppm residual uncertainty, the alarm clock would even be precise to 8 seconds per year worst case.

For the ALV-7002/USB correlator such a high stability clock generator can be ordered as option, for all other ALV correlator products this option will become available soon. The high stability clock generator offers the following performance:

• better than 0.25 ppm initial frequency accuracy (measured at room temperature and after 24 hours of operation)
• less +/-0.5 ppm frequency drift over the entire operating temperature range
• very low phase noise
• less than +/-1 ppm ageing (first year, typically less from year two on. Factory pre-ageing available on request and on surcharge)
• GPS/”stable frequency input” based ageing trim with +/- 5 pm range
• the clock generator can additionally be used to output a highly stable 10 MHz clock at on one of the user I/O pins of the correlator. The frequency output which will be precisely in phase with the correlators sampling time with some constant phase difference of a few ns due to pulse propagation

GPS Based Ageing Trim

A special software package comes along with the above outlined clock generator option. Using this and feeding the PPS (pulse per second) output of a GPS receiver into the correlator input the exact frequency of the clock generator can be determined and ageing effects can be compensated via a trimmer accessible from the below side of the correlator housing.  

Typically commercial grade GPS receivers with PPS output ensure a PPS stability of better than 1 µs (1 ppm), special GPS receivers designed for timing applications ensure a PPS stability of better 25 ns (0.025 ppm). While in the first case, measurement times of a few 10 seconds must be expected to average the (random) uncertainty in the PPS pulse, the use of a high precision GPS receiver allows practically “real-time” measurements of the actual clock generator frequency with updates every second. Alternatively, a stable high frequency clock (a frequency standard, for example) with either 5 or 10 MHz frequency can be fed to the correlator as well. If so, a frequency stability for the frequency standard of better 0.1 ppm is required.

Using this special software package and a GPS receiver with PPS output (or a frequency standard), the user can easily re-trim the stable clock generator to better 0.25 ppm absolute on-site within just a few minutes.